1. Field of the Invention
The present invention relates to a power supply control unit and a power supply control method, and more particularly to a power supply control unit having a semiconductor switch for controlling a power supply from a power source to a load by switching control depending on a control signal.
2. Description of the Related Art
FIG. 1 shows a conventional power supply control unit having a semiconductor switch. This conventional power supply control unit supplies a power from a battery to each load selectively and controls the power supply to the load.
In this conventional power supply control unit shown in the same Figure, a shunt resistor RS and a drain D and source S of a thermal FET QF are connected in series in a path for supplying an output voltage VB of a power source 101 to a load 102 such as a head lamp and a power window driving motor. Further, this power supply control unit comprises a driver 901 for detecting a current flowing in the shunt resistor RS so as to control a driving of the thermal PET QF by a hardware circuit , an A/D converter 902 for carrying out ON/OFF control of a driving signal of the thermal FET QF based on a current value monitored by a driver 901, a microcomputer (CPU) 903. The thermal FET QF acting as a semiconductor switch contains an over-heat shut-off function for turning itself OFF by an incorporated gate shut-off circuit when an incorporated temperature sensor (not shown) detects that a temperature of the thermal FET QF rises up to a temperature above a predetermined one. A symbol RG in the same FIG. indicates an incorporated resistor and ZD1 denotes a Zener diode which maintains 12 V between gate G and source S and bypasses when an overvoltage is about to be applied to the gate G.
Further, this conventional power supply control unit also includes a protective function for an overcurrent on the load 102 or between the drain D and source S of the thermal FET. Namely, the driver 901 comprises differential amplifiers 911, 913 as a current monitor circuit, a differential amplifier 912 as a current control circuit, a charge pump circuit 915 and a driving circuit 914 for driving the gate G of the thermal FET QF via an internal resistor RG based on an ON/OFF control signal from the microcomputer 903 and a result of overcurrent determination from a current control circuit.
When a voltage different from the output voltage VB of the power source 101 is used, for example, VB is 42 V while the other necessary voltage is 12V, it is necessary to provide a new power supply control unit including the aforementioned charge pump 915. By providing the power supply control unit for 12 V, 12 V can be obtained as well as 42 V.
When an overcurrent is detected via the differential amplifier 912 based on a voltage drop in the shunt resistor RS by determining that the currant exceeds its predetermined (upper limit) value, the thermal FET QF is turned OFF by the driving circuit 914 and when the current drops below a predetermined (lower limit) value, the thermal FET is turned ON.
On the other hand, the microcomputer 903 always monitors a current via current monitor circuits (differential amplifiers 911, 913). When an abnormal current exceeding a normal value flows, it turns OFF the thermal FET QF by turning OFF a driving signal to the thermal FET QF. When the temperature of the thermal FET QF exceeds its predetermined value before the driving signal for OFF control is outputted from the microcomputer 903, the operation of the thermal FET QF is turned OFF by the overheat shut-off function.
However, in the above-mentioned conventional power supply control unit, a charge pump is necessary for that control. Because generally, the charge pump circuit is large in size, reduction of the size of the charge pump and its production cost are difficult.
Further, when the charge pump is necessary, the total number of components of the power supply control unit increases and the circuit thereof becomes complicated. Thus, a reliability of such a power supply control unit is not so satisfactory.
Further, to detect a current, a shunt resistor RS connected to a power supply path in series is necessary. Because, in recent years, current flowing to the load has been increased with a reduction of ON resistance of the thermal FET QF, heat loss of the shunt resistor cannot be neglected.
Although the aforementioned overheat shut-off function and overcurrent limiting circuit function when a substantially complete short-circuit occurs in the load 102 and wiring so that a large current flows, they do not function when a rare short-circuit such as incomplete short-circuit having some extent of short-circuit resistance occurs so that a small short-circuit current flows. Therefore, there is no way but turning OFF the thermal FET QF by detecting an abnormal current by a microcomputer 903 via a current monitor circuit. However, according to the conventional art, a response of microcomputer control to the abnormal current is not satisfactory.
Further, because the shunt resistor RS. A/D converter 902, microcomputer 903 and the like are necessary, a large installation space is necessary. Thus, there is a problem that the cost for the unit is increased by these relatively expensive components.
The present invention has been achieved to solve the above described conventional problems and therefore, an object of the invention is to provide a power supply control unit having a simple circuit structure and a high reliability in which a charge pump circuit can be eliminated,
Another object of the invention is to provide a power supply control unit and power supply control method in which a shunt resistor connected to a power supply path for detecting a current in series is unnecessary so as to suppress heat loss and a quick response is enabled against an abnormal current generated when a rare short-circuit such as incomplete short-circuit having some extent of short-circuit resistance occurs, the power supply control unit being easy to integrate and not expensive.
To achieve the above object, there is provided a power supply control unit comprising: a power transmission means for supplying a power from a power source to a first load; an auxiliary power means for generating at least a kind of voltage different from a voltage of the power source; a semiconductor switch for controlling a power supply to a second load from the auxiliary power means by switching control depending on a control signal supplied to a control signal input terminal thereof; a reference voltage generating means for generating a reference voltage having a voltage characteristic equivalent to the voltage characteristic of a voltage between terminals of the semiconductor switch when a predetermined load is connected thereto; a detecting means for detecting a difference between a voltage between the terminals of the semiconductor switch and the reference voltage: a control means for controlling ON/OFF of the semiconductor switch depending on a difference between the detected voltage between the terminals and the reference voltage; and a control reference voltage supply means for supplying a voltage serving for a reference of control to the control means.
According to the present invention, an electric power is supplied to both the first load requiring a predetermined voltage of a power source and a second load requiring a voltage different from the predetermined voltage. The power supply to the second load is carried out through a current vibration type shut-off function provided switching circuit. As a result, an abnormal current such as a rare short-circuit, ordinary short-circuit and excessive current can be detected by monitoring current. When the abnormal current is detected, the power supply can be Interrupted.
According to a preferred embodiment of the present invention, the reference voltage generating means comprises a circuit which is connected in parallel to the semiconductor switch and the second load and in which a second semiconductor switch controlled by switching depending on the control signal and a third load are connected in series, said reference voltage generating means generating a voltage between terminals of the second semiconductor switch as the reference voltage.
According to another preferred embodiment of the present invention, the voltage characteristic possessed by the reference voltage of the reference voltage generating means is equivalent to the voltage characteristic In a condition that a target current which is maximum current in normal operating range flows to the semiconductor switch and the second load.
According to still another preferred embodiment of the present invention, the semiconductor switch and the second semiconductor switch have equivalent characteristic with respect to a transient voltage characteristic of a voltage between the terminals at the time of change from OFF state to ON state.
According to the above described embodiments, the necessity of the conventional shunt resistor is eliminated so as to suppress heat loss. Further, not only an overcurrent due to complete short-circuit but also an abnormal current generated when a rare short-circuit such as Incomplete short-circuit having some extent of short-circuit resistance can be detected continuously by a hardware circuit or programming on microcomputer. Specifically when the ON/OFF control of the semiconductor switch is achieved by a hardware circuit, the microcomputer is also unnecessary. Therefore, the installation space can be reduced and the production cost of the power supply unit can be reduced largely.
According to a further preferred embodiment of the present invention, a current capacity of the second semiconductor switch is smaller than the current capacity of the semiconductor switch and a ratio of resistance between the second load and the third load is equivalent to a ratio of the current capably between the semiconductor switch and the second semiconductor switch.
According to this embodiment, the circuit structure of the reference voltage generating means having the second semiconductor switch and the third load can be reduced in size, so that the installation space can be reduced and the cost on the power supply control unit can be also reduced.
According to a further preferred embodiment of the present invention, the third load has plural resistors and a resistance of the third load is changed and set by connecting selectively any one of the plural resistors.
According to a further preferred embodiment of the present invention, the aforementioned power supply control unit further comprises a variable resistor connected to the second load in series or connected to the third load in parallel wherein a resistance of the third load is changed and set by the variable resistor.
According to the above two embodiments, plural specifications can be covered by a single kind of the chip. Complete short-circuit and incomplete short-circuit can be detected separately depending on the type of the load securely, so that protection against short-circuit fault can be carried out accurately.
According to a further preferred embodiment of the present invention, the control means turns OFF the semiconductor switch when a difference between a detected voltage between the terminals and the reference voltage exceeds a first threshold value and turns ON the semiconductor switch when a difference between the detected voltage between the terminals and the reference voltage drops below a second threshold value.
According to a further preferred embodiment of the present invention, the aforementioned power supply control unit further comprises an overheat protecting means for protecting the semiconductor switch by OFF control when the semiconductor switch is overheated.
According to this embodiment, when an incomplete short-circuit having some extent of short-circuit resistance occurs, the ON/OFF of the semiconductor switch is repeated by the control means so as to change current largely. The shut-down of the semiconductor switch by the overheat protecting means is accelerated by cyclic heat generation of the semiconductor switch, so that a quick response to abnormal current generated when the incomplete short-circuit occurs can be achieved.
According to a further preferred embodiment of the present invention, the semiconductor switch, the reference voltage generating means, the detecting means, the control means and the overheat protecting means are formed on a same chip.
According to this embodiment, the circuit structure of the power supply control unit can be reduced in size, so that the installation space can be reduced and the cost on the power supply control unit can be reduced and further, an influence of temperature drift and deviation between lots can be eliminated.
According to a further preferred embodiment of the present invention, a cycle of ON/OFF control of the semiconductor switch by the control means is used as a control clock,
According to this embodiment, an oscillation circuit dedicated for the control clock becomes unnecessary. Because the cycle of the ON/OFF control of the semiconductor switch (FET) is stabilized, a stabilized clock can be obtained as the control clock.
According to a further preferred embodiment of the present invention, the aforementioned power supply control unit further comprises an inhibit means for Inhibiting ON/OFF control of the semiconductor switch by the control means in a predetermined period after the semiconductor switch is turned to ON state.
According to this embodiment, overcurrent control which may occur when rush current flows upon a rise-up of the second load is Inhibited so as to suppress a delay of load response.
According to a further preferred embodiment of the present invention, the power supply control unit further comprises an overheat shut-off accelerating means for accelerating OFF control by the overheat protecting means during ON/OFF control of the semiconductor switch by the control means.
According to this embodiment, the shut-off of the semiconductor switch upon an incomplete short-circuit is accelerated so as to achieve a quick response.
According to a further preferred embodiment of the present invention, the power supply control unit further comprises a frequency control means for Integrating ON/OFF control frequency of the semiconductor switch by the control means and when the control frequency reaches a predetermined frequency, turning OFF the semiconductor switch.
According to this embodiment, a quick response can be achieved by accelerating the shut-off of the semiconductor switch even upon an incomplete short-circuit up to an arbitrarily set time.
Further, to achieve the above object, there is provided a power supply control method using a power supply control unit comprising: a power transmission means for supplying a power from a power source to a first load: an auxiliary power means for generating at least a kind of voltage different from a voltage of the power source; and a semiconductor switch for controlling a power supply to a second load from the auxiliary power means by switching control depending on a control signal supplied to a control signal input terminal thereof, the power supply control method comprising: a reference voltage generating step for generating a reference voltage having a voltage characteristic equivalent to the voltage characteristic of a voltage between terminals of the semiconductor switch when a predetermined load is connected thereto; a detecting step for detecting a difference between a voltage between the terminals of the semiconductor switch and the reference voltage; a control step for controlling ON/OFF of the semiconductor switch depending on a difference between the detected voltage between the terminals and the reference voltage; and a control reference voltage supply step for supplying a voltage serving for a reference of control to the control means.
According to a preferred embodiment of the present invention, in the reference voltage generating step, the voltage characteristic possessed by the reference voltage is equivalent to the voltage characteristic in a condition that a target current which is maximum current in normal operating range flows to the semiconductor switch and the second load.
According to another preferred embodiment of the present invention, the control step comprises an OFF control step for turning OFF the semiconductor switch when a difference between a detected voltage between the terminals and the reference voltage exceeds a first threshold value; and an ON control step for turning ON the semiconductor switch when the difference between the detected voltage between terminals and the reference voltage drops below a second threshold value.
According to still another preferred embodiment of the present invention, the power supply control method further comprises an overheat protecting step for protecting the semiconductor switch by OFF control when the semiconductor switch is overheated.
According to a further preferred embodiment of the present invention, the power supply control method further comprises an inhibit step for inhibiting ON/OFF control of the semiconductor switch by the control step in a predetermined period after the semiconductor switch is turned to ON state.
According to a further preferred embodiment of the present invention, the OFF control by the overheat protecting step is accelerated at the time of ON/OFF control of the semiconductor switch by the control step.
According to a further preferred embodiment of the present invention, the power supply control method further comprises a frequency control step for integrating ON/OFF control frequency of the semiconductor switch by the control step and when the control frequency reaches a predetermined frequency, turning OFF the semiconductor switch.
The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.